Long time passing. 
Sharon Levy’s Once and Future Giants: What Ice Age Extinctions Tell Us About The Fate of Earth’s Largest Animals is hard to put down. It’s sort of a Pleistocene CSI: 13,000 years later, scientists are trying to put together the pieces of what happened to the large animals of North America. These include giant short-faced bear, dire wolf, mastodon, saber toothed cat, mammoth, giant ground sloth, tapir, giant beaver, lion, camel, stag moose, horse, and more. It was once thought that climate change did them in. Then it was argued that human hunting, overkil, did the deed, for the animals disappeared in rather a short time just after the first humans arrived in the Americas. Now the best bet on this kind of thinking may be a synthesis of both climate change/habitat shift and expanding human population. The animals had, after all, survived many interglacial warming periods between ice ages before. What they hadn’t meet before were humans, the technologically-armed predator (cf. Clovis points). Weakened by environmental stresses caused by a warming planet, the animals, usually (like the elephant today) slow to mature and slow to breed, became vulnerable at the species level to expanding humans. And the animals that predated the big herbivores were deprived to food, and around it went, the cycle of extinction.
The story is not unique to the New World. About 45,000 years ago, the giant mammals of Australia began to disappear. This was just after humans reached there. The story is repeated on big islands the world over. And on the largest of the landmasses, Eurasia. Finally, humans got to New Zealand, only about 750 years ago, and within a short period all nine species of flightless moas, some of which towered over people, were gone. When Europeans arrived there, they found nothing but the bones of thousands of these birds.
African megafauna survived best, possible because they had evolved with humans for a longer period of time. Now, of course, with the pressures of human population pressing them on all sides, they are severely threatened. They are the last of the megafauna.
What is particularly interesting about Levy’s book is the way she elucidates the complexities of competing theories and faint evidence, and the interwoven story about how these giant mammals changed their habitats, and how, with them absent, these habitats have suffered since. Transporting seed, depositing fertilizer, breaking up the ground with their hooves, browsing grasses or shrubs (depending on species), they created habitat which has since disappeared or been severely reduced. The giant beavers, like their little, surviving, cousins, were engineers of wetlands on a continental scale. (Levy doesn’t mention this, but the near extinction of the North American beaver by the European fur trade from the 17th century onwards radically changed the face, and nature, of the continent.)
Large predators also turn out to be key regulators of habitat, as the reintroduction of wolves has shown in the American West. They prey on the herbivores who would otherwise deforest the area. Dingoes in Australia — descended from Indian wolves — are another good example; they regulate the mesopredators, those mid-level predators (in Australia’s case, introduced foxes and feral cats) that wreck havoc without something policing them. Raccoons and feral cats are the equivalent North America plague.
Both wolves and dingoes are of course controversial cases; the ruralists in both the U.S. and Australia are very much against them. The struggles to reintroduce wolves in the U.S. and to keep Aussies from slaughtering dingoes are real world brakes to “Pleistocene rewilding.” This means introducing similar animals to the extinct species — elephants, camels, etc. — in places that haven’t seen anything like them in 13,000 years, so they can help to bring back some semblance of environmental order to the chaos. (Invasive introductions have a rather poor track record, though.) Of course, with big herbivores, you need big predators, and it seems unlikely that many people are going to want to see lions reintroduced to Wyoming any time soon. The introduction of horses back to the New World by Spaniards is a case study, inadvertent as it is, of such rewilding.
You should definitely read this book for more details, for in the study of the past there may be keys to the future, and saving the world’s last remaining megafauna.
Skeleton of Columbian mammoth in the George C. Page Museum at the La Brea Tar Pits, Los Angeles, CA. Image by WolfmanSF, Wikimedia Commons.
